This invention relates generally to digital electronic circuits and more particularly to digital integrated circuits that determine which signal arrived first.
Arbiters are used in digital electronics to determine which signal arrived first. This function has many uses in modem electronics. Some of the se uses include resource arbitration where two devices compete for a single resource (such as a memory, or signal bus) by sending a signal to an arbiter. The arbiter determines which device signaled for the resource first, and may then award that device use of that resource. Another example of the usefulness of arbiters is for phase comparison. An arbiter, since it indicates which of two signals arrived first, is useful as a phase comparator. An excellent example of this application is disclosed in a U.S. Pat. No. 6,323,714, titled SYSTEM AND METHOD FOR DESKEWING SYNCHRONOUS CLOCKS IN A VERY LARGE SCALE INTEGRATED CIRCUIT, which is hereby included by reference for all that is disclosed therein. This patent is commonly owned by the assignee of the present document and was filed in the United States Patent and Trademark Office on about the same day as the present application.
One problem with arbiters in general is the occurrence of a metastable condition. If both signals arrive together or very close together, the arbiter will generally enter a metastable state. When this occurs, the outputs may move to voltage levels that are not valid logic levels. These voltage levels may be called metastable voltage levels. This condition may last for an arbitrary period of time. Another problem that may arise is that some arbiter circuits may have outputs that can oscillate when the inputs oscillate. In other words, the outputs may not be steady between arbitrations. Both of these conditions, non-valid logic levels and oscillating outputs, can cause problems with the circuitry that receives them.
Accordingly, there is a need for a way to realize an arbiter that maintains a steady output state between arbitrations and yet never propagates a metastable voltage level to its outputs.
In a preferred embodiment, the invention provides an arbiter that maintains stability between arbitrations without propagating metastable voltage levels. This arbiter is well suited for fabrication in monolithic integrated circuits as well as other circuit technologies. In addition, the preferred embodiment is implemented using a complementary metal oxide semiconductor (CMOS) devices. CMOS devices and processes are commonly used in many VLSI or other integrated circuit designs, so this embodiment may be used in these devices.
The arbiter of the present invention includes a phase comparator receiving two input signals. The outputs of the phase comparator are propagated to a first SR type flip-flop. The outputs of the first SR type flip-flop are propagated to a second SR type flip-flop. The outputs of the second SR type flip-flop indicate which of the two input signals changed first.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.